1. Field of the Invention
This invention relates generally to the field of clean coal energy production. More specifically, the invention relates to a method of combining two clean coal processes into a single facility.
2. Background of the Invention
Natural gas, coal deposits, and biomass are abundant energy sources that often serve as fuel for power generation. The United States has significant known reserves of coal, and coal currently burned for power generation represents only a fraction of the total deposits that are available. This coal used for power satisfies approximately one-half of the electrical energy demand of the United States. Current and tightening emissions regulations limit the amount of sulfur, oxides of nitrogen, and greenhouse gas emissions, making coals high in these materials less favorable for electricity generation. Failure to comply with these regulations results in hefty penalties in the form of fines, shutdowns, and limited operations. Further, upgrading of current coal burning electric generation plants requires substantial investment. With increased awareness of environmental issues such as global warming, and greenhouse gases, this trend is predicted to continue, potentially making coal-burning plants unfavorable and expensive to operate.
Rising worldwide oil demand has increased the cost of oil distillates and encouraged development of alternative clean power facilities. One such power production facility is the Integrated Gasification Combined Cycle (IGCC) plant. Electricity is produced from the combustion of a synthesis gas (syngas) produced by the gasification of coal. Gasification is a method of reacting coal with limited oxygen at high temperatures for the production of synthesis gas. The process of gasification removes potential pollutants such as mercury, arsenic, nitrogen oxides, and sulfur oxides. Further benefits are realized when syngas is combusted, as the burning of syngas releases lower amounts of carbon dioxide. The lowered carbon dioxide emissions and technological advances make these facilities “capture ready,” since stored carbon dioxide from the coal is removed in the gasifier.
Conventional steam turbines require pressure, temperature, and corrosion resistant components to generate electricity. The limitations of these components dictate the upper range of the steam temperatures, and therefore limit efficiency of electric power production. In the case of an IGCC gas turbine, the same limitations do not apply. The gas turbine has a higher gas cycle firing temperature that feeds the compressor, burner, and turbine systems as a means of electricity production. The high-temperature exhaust-gas output of the turbine can be used to heat steam for a supplemental steam turbine, thereby increasing the overall efficiency of an electrical power plant.
Coal may also be converted to a synthetic liquid fuel by the conversion of syngas. Liquid fuels have an advantage over coal in that they are easily transported long distances without expensive processes or packaging. The process of converting coal to a liquid fuel typically involves a catalytic reaction of syngas to form liquid hydrocarbons. Fischer-Tropsch (FT) reactor facilities execute the vital step of catalyzed synthesis of petroleum substitute liquid fuels. The process occurs via a catalyzed chemical reaction in which the carbon monoxide and hydrogen in syngas are converted into liquid hydrocarbons. The production of liquid hydrocarbon fuels from solid material reduces dependence on oil distillates for fuels. The hydrocarbon production reaction is highly exothermic, and requires a cooled reactor to maintain conditions favorable for continued synthesis.
The IGCC and CTL process represent two potential clean coal processes to reduce dependence on oil distillates. The former provides clean electrical power and the latter provides liquid hydrocarbons for further processing into products. Additionally, both processes require coal processing, air separation, and syngas production for operation. The current costs of material, process, capital, and infrastructure make individual investments in these processes expensive rendering them unfavorable for development. Previous discussions on the combination of facilities for these processes have centered on their shared starting material and parallel infrastructure requirements for the production of syngas. However, it is recognized that providing a singular source of syngas to operate a plurality of clean coal plants is disadvantageous for operational flexibility, the capacity to scale output to demand, and maintain production during maintenance, or in the case of a device failure.
Accordingly, there is a need in industry for a method of integrating IGCC and CTL facilities with operational flexibility, scalable output, and online maintenance.